The invention relates to a high-lift system for an aircraft with a main wing and a slat that, by means of an adjustment device, can be adjusted relative to said main wing to various adjustment states, with said high-lift system comprising a device for influencing the airflow. By means of the device for influencing the airflow, in particular a reduction in the aerodynamic noise on slats of aircraft is achieved.
Documents U.S. Pat. No. 3,363,859, EP 0230684A and DE746714C disclose a high-lift system for an aircraft with a main wing and a slat that by means of an adjustment device is adjustable relative to said main wing to various adjustment states, with a gap resulting between the rear of the slat, wherein at the rear of the slat an air outlet is formed.
In order to be able to generate sufficient lift force during takeoff and landing, the aerofoils of modern aircraft mostly comprise variable high-lift devices (slats and flaps) whose extension results in the adjustment of a base geometry, which is optimised to cruising, to low flight speeds. In the extended state there is a gap between the slat and the main wing, through which gap the air is accelerated and moves with considerable sound emission from the pressure side of the wing to its suction side. Presently the flow around the lift-generating surfaces is a significant contributor to noise generation of aircraft in the landing approach.
The airflow around the slat creates a recirculation region which on the one hand is bordered by the slat pressure side in the form of a channel, and on the other hand is separated from the fast gap airflow by a free shear layer. Instabilities in this free shear layer result in the formation of discrete vortex structures which are then conveyed continuously along a (an imaginary) separation flow line in the direction of a second stagnation point on the underside of the slat. At this position the airflow splits, and the vortex structures partly enter the recirculation region, and partly exit, greatly accelerated, through the gap between the slat and the main wing. The accelerated movement of the vortices and in particular their exit from the gap result in the emission of sound waves, wherein the sudden impedance jump between the solid wall and the free airflow when flowing over the trailing edge of the slat results in generating acoustic fluctuations as a result of hydrodynamic pressure fluctuations. If the trailing edge of the slat is obtuse, the interaction of the emanating vortices with a vortex path that results further downstream should be mentioned as an additional source of noise.
Measures relating to noise reduction can be aimed at adjusting the geometry of the slat to the shape of the recirculation region (displacement body, separating surface, bellows). Likewise, devices for influencing the free shear layers (rows of broom fibre) or for attenuating sound propagation (acoustic absorber surfaces) can be provided.
One option relating to the aerodynamic noise reduction of slats of a commercial aircraft is known from DE 100 19 185 A1. In this arrangement the rear profile surface of the slat, the shape of which profile surface matches the exterior contour of the main wing, comprises a hollow displacement body (bellows) that can be inflated by means of compressed air from at least one bleed air line. If the bellows is pressurised while the slat is extended, the bellows expands and the size of the adjoining recirculation region decreases. With a suitable shape in the inflated state, noise-emitting vortex formation at the extended slat is thus reduced.
DE 199 25 560 A1 describes the incorporation of a rigid or flexible separating surface attached to the additional wing, which separating surface is arranged along the separation flow line that extends between the return flow region and the gap flow, and extends in the direction of the main wing, as a result of which arrangement the pulse exchange transversely to the direction of the gap flow can be impeded, and consequently the sound-source level of slats can be reduced.
A similar method for reducing the aerodynamic noise on an additional aerofoil of an aircraft is described in DE 10 2004 056 537 A1. The arrangement comprises an n-stable separating surface that by means of an actuator device, when the additional aerofoil is in the extended state, can be displaced to the gap and in that location entirely or partially extends along the separation flow line situated between the recirculation region and the gap flow, as a result of which displacement the vortex formation and ultimately the sound radiation can be significantly reduced.
From DE 101 57 849 A1 a further arrangement for reducing the aerodynamic noise on a slat of a commercial aircraft is known, which arrangement reduces the energy exchange by way of the shear layers that arise on the extended slat with the use of several broom fibres arranged in series along the slat edges. The final flowthrough resistance of these rows of broom fibre that form a separating surface results in a more gentle equalisation of the turbulent alternating pressures in the direction of the airflow, and ultimately results in weakening of the noise source mechanisms active in the shear layers.
U.S. Pat. No. 6,454,219 describes an arrangement in which with the use of sound-absorbent materials on the slat and/or on the main wing the sound waves are attenuated while still in the gap region, as a result of which the outwards-radiated acoustic energy is reduced.
In arrangements which for the purpose of noise reduction provide for the positioning of rigid structures in the gap between the slat and the main wing (e.g. a rigid separating surface) there is generally a danger that in the case of a defect in the kinematic actuator mechanism required for retraction the articulation of the slat to the main wing is prevented so that the aircraft needs to remain in its high-lift configuration.
The use of additional mounting parts is associated with a disadvantage in that it basically results in increased maintenance expenditure. This will be necessary in particular in the case of elastic or movable subassemblies in order to prevent possible destruction as a result of aging or fatigue of the material used, wherein fatigue can be caused either by fluctuating aerodynamic loads or by design-related alternating loads.
In the case of components aimed at matching the slat contour to the shape of the recirculation region, undesirable aerodynamic effects can occur as a result of sudden changes in the flow conditions (e.g. angle of attack) unless immediate matching of the contour to the changed boundary conditions takes place.
Arrangements that aim to attenuate propagation of noise cannot reduce sound radiation to the extent theoretically possible by favourably influencing of the sound mechanism.